Book/Report

FZJ-2019-05086

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Wachstum kleiner Risse bei hohen Temperaturen und Zug-Schwellbeanspruchungen in den einkristallinen Superlegierungen CMSX-4 und SC16

Rieck, T. (Corresponding author)FZJ* ; Schubert, F.FZJ*

1999
Forschungszentrum Jülich, Zentralbibliothek, Verlag Jülich

Please use a persistent id in citations: http://hdl.handle.net/2128/23108

Report No.: Juel-3706

Abstract: CMSX-4 is a single crystal candidate superalloy for the highly stressed blades and vanes of a heavy stationary gas turbine. It is assumed that the failure of a turbine blade in service is caused by growth of fatigue or creep-fatigue cracks < 1 mm long in the aerofoil or root regions. In this work the fatigue and creep fatigue crack growth behaviour was investigated using single-edge-notched (SEN) specimens with edge and corner crack geometries at 750 and 1000°C. In addition single crystal alloy SC16 with reduced '$\gamma$' content was included in the test programme. The $\Delta$K threshold values for fatigue crack growth were for both alloys higher at 750 than at 1000°C. Futhermore the corner-crack specimens exhibited lower threshold values and a more pronounced growth of small cracks than edge-crack specimens. For edge-crack specimens at 750°C, the <100> crack orientation had a lower threshold value, and a slightly higher crack growth rate in the Paris region, than the <110> orientation. This anisotropic behaviour of the alloys was more marked for CMSX-4 than for SC16. Examination of the macro fracture surfaces showed that in the <100> orientated edge- crack specimen the crack followed the „weak" <100> direction. When the crack was initiated in the <110> direction, it was deflected and propagated in the <100> direction. The stress intensity factors for CMSX-4 and SC16 calculated by a FE-method showed differences between isotropic and anisotropic materials behaviour. These differences increased with an increasing degree of anisotropy. From the microscopical point of view, at 750°C and low $\Delta$K values the crack grew along {100} planes. At higher $\Delta$K values, the crack path was zigzag-shaped with a change of the crack path between {111} and {100} planes. At higher degrees of deformation or high $\Delta$K values and crack growth rates, the $\gamma$ ' particles were cut. Therefore rapid fracture along {111} planes was observed. At 1000°C in CMSX-4 the crack growth mainly followed the $\gamma/ \gamma$' interface on {100} planes. At high temperatures the $\gamma$' could only be cut at very high degrees of deformation rates. SC16 did not show the change of the crack path to {111} planes. This can be explained by the difference in the $\gamma$' content between CMSX-4 and SC16. The crack growth was only influenced by the initial raft-like $\gamma$' structure at 750°C. The crack initiation point or the initial stage of crack growth was influenced by oxidation, the oxidation rate being higher than the crack growth rate. Therefore the crack stopped or slowed down. In the later stages of crack growth, the influence of oxidation decreased. A comparison of the results of crack growth experiments in air and in vacuum showed exactly this influence of oxidation.

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Contributing Institute(s):

1. Publikationen vor 2000 (PRE-2000 ; Retrocat)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)

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